As I said back on page one, the guy who sculpted the helmet originally, did so as close as he could over a cast of his own head, which he said was on the large side. Your head circumference puts you on the line between small and medium hat sizes. I'm about an inch bigger around, and solidly in the "Large" size category, but I also have what milliners call a "long" skull shape -- longer front to back than side to side. This, as opposed to the "standard" head shape that is, while not exactly symmetrical, more balanced between the two. So I'd say large to extra large, depending on head shape, would fit an accurate one of these. So it's no wonder you find them too big.
I know they're pricey, but have you ever tried on a Don Post RG helmet from the '90s?